1. Field of the Invention
The present invention relates to switches generally. In particular, it relates to pull-out fusible switches having the ability to interrupt high-current DC circuits by extraction of a fuse.
2. Related Art
Switches that are capable of interrupting high-current DC circuits are utilized in numerous environments. One such use is in the telecommunications industry. The DC side of a telecommunications power plant often includes circuits attached to high amp-hour battery strings that provide back-up DC power. Switches that operate in such circuits are often required to withstand up to 100,000 Amps of short-circuit current.
DC switches previously used in such applications had power fuses that were bolted inside the switch. Replacement of a blown fuse in such a device requires the use of metal tools, such as wrenches, to loosen the nuts that secure the fuse to the switch. The potential for a metallic tool or a piece of metallic hardware coming into contact with a copper busway, rectifier, or battery compartment poses the risk of causing the equipment to short-circuit. Existing DC switches are also bulky, complicated, and expensive.
U.S. Pat. No. 3,418,615 to Canney discloses a removable fuse holder for converting unfused transformers into fused transformers. The fuse holder of Canney includes a fuse container with a cover member that fits on the container. The cover includes a pair of fuse brackets that carry a pair of cylindrical fuses. The relatively thin ferrule-type terminals of the cylindrical fuses are not well suited for the frictional wear associated with repeated removal of the fuse from the fuse clips while switching. A pair of fuse clips are mounted to each compartment of the fuse container. One fuse clip has a bar-like projection that extends out through a side surface of the container for electrical connection to a terminal of a transformer. The other fuse clip has terminal member that also projects out through a side surface of the container. The additional bar member and terminal member represent additional electrical contacts. Additional electrical contacts are disadvantageous because they result in increased resistance and the associated increase in operating temperature, as well as higher voltage drops (or watt losses).
U.S. Pat. No. 4,536,046 to Erickson discloses a pull-out fusible switch for use in circuit-breaker boxes. The switch includes a base and removable cover. The base includes terminal contacts that extend out of the side of the base. Fuse clips are mounted to the base and are used to retain cylindrical fuse elements. The fuses are mounted within the base member and remain within the base when the cover is removed. The cover has a downwardly extending blade that makes an electrical connection with the base. Separate bar-like members provide an electrical connection between the fuse clips and the side terminals. As noted above, these additional electrical contacts are not desirable.
U.S. Pat. No. 3,139,499 to Cosgrove discloses a fuse adapter capable of replacing circuit breakers. The adapter has a body with a removable cover. The cover includes fuse clips that carry a cylindrical fuse element. The cover also has a lamp that indicates whether a fuse has blown. Separate electrical contacts are mounted to the base and electrically connect the fuse to terminals via wire leads.
U.S. Pat. No. 2,091,204 to Horn discloses a fuse box that can be inserted into a three-phase power supply to an electric power tool. The fuse box includes a base and a removable top. The top has a fuse carrier clip member that carries a cylindrical fuse. The fuse is electrically connected to side terminals by separate U-shaped contact clips and a bar-like member screwed to each contact clip.
U.S. Pat. No. 3,272,947 to Kobryner discloses a pull-out switch arrangement. The switch has a tub or base and a puller or top. The top has fuse clips that retain the terminals of a fuse member. Separate flange and bus bar fingers are screwed to the base and electrically connect the fuse to the bus bar when the top is inserted into the base. Once again, the additional electrical contacts formed by this construction results in increased resistance and the associated increase in operation temperature, as well as high voltage drops (or watt losses).
U.S. Pat. Nos. 4,966,561 and 5,186,637 to Norden each discloses a fuse holder having a receptacle and a fuse carrier. Separation of the fuse carrier from the base is facilitated by a cam-action lever. The fuse carrier has a pair of terminal brackets that contact the ends of a cylindrical fuse element. A separate U-shaped contact is screwed to the terminal brackets. Each leg of the U-shaped contact is held by a pair of contact fingers which are mounted to the receptacle, thereby electrically connecting the fuse to the receptacle by multiple electrical contacts. A separate clamp member receives an electrical connector inserted through an aperture in the side of the receptacle. The resulting construction is complicated and has multiple electrical connections.